This invention relates to a pressure sensor apparatus, and more particularly to a thin, flexible device that allows measurements relating to the position of pressure on the sensor to be determined.
There are many application in which it is desirable to sense pressure between two objects. For instance, it is often necessary to sense pressure between nipped rollers employed in the paper making and converting industries, as discussed in detail in Applicant""s co-pending application, U.S. patent application Ser. No. 09/239,610, which is incorporated by reference herein as fully as if set forth in its entirety.
Force sensing resistors are employed in the prior art in order to measure an amount of force between objects. FIG. 20 illustrates one example of a force sensing resistor (e.g.xe2x80x94a linear potentiometer), having two polymer films or sheets. On one sheet, a conducting pattern is deposited on the polymer in the form of interdigiting electrodes 2. Semi-conducting polymer 1, which is a pressure-responsive resistive medium having a force/resistance characteristic such as illustrated in the graph of FIG. 21, is deposited on the other sheet. The pressure responsive resistive medium, as shown in FIG. 21, has a resistance that lowers as the pressure exerted thereon increases. When the two sheets of the force sensing resistor are brought together, the resistance between the interdigiting electrodes is very high. However, when a force is applied, the resistance of semi-conducting polymer 1 reduces sufficiently to shunt an electrical conduction at the interdigiting electrodes where the force is applied.
Force and position sensing resistors, which may also comprise a linear potentiometer, are employed in the prior art to determine a position of an applied force. FIG. 22 illustrates an example of such a linear potentiometer, having fixed resistor strip 13 with hot end 15 and ground end 14. Wiper 12 comprises a plurality of conducting fingers as shown. Force sensing layer 11 is brought together with wiper 12, such that when a force is applied to force sensing layer 11, the wiper contacts are shunted through force sensing layer 11 to the conducting fingers at the location of the applied force. A voltage read across wiper 12 is proportional to the distance along the device that the force is applied.
Another type of force and position sensing resistor that exists in the prior art is an XYZ pad, which is illustrated in FIG. 23. The XYZ pad shown incorporates the same basic features as a linear potentiometer, except that the XYZ pad employs two such arrangements so as to measure the position of an applied force in a plane. Specifically, force sensing layer 11 is double sided and is disposed between the conducting fingers of wiper 12a arranged in one direction, and the conducting fingers of wiper 12b arranged in a perpendicular direction. On each side of the pad, a force applied to double-sided force sensing layer 11 results in the wiper contacts being shunted through force sensing layer 11 to the conducting finger adjacent to the location of the applied force. A voltage read across wiper 12a is thus proportional to the distance along the x-axis of the device that the force is applied, while a voltage read across wiper 12b is proportional to the distance along the y-axis of the device that the force is applied. By determining the distance along the x- and y-axes, the position of the applied force within a plane is defined.
One problem with traditional force sensing resistors and the force and position sensing resistors in the prior art is that they are inaccurate. For instance, a force applied to a prior art sensor may inadvertently shunt an electrical conduction through an area that is larger than the particular area that pressure is applied to. This occurs because, when a force is applied to the force sensing layer, the force sensing layer contacts the conducting fingers of the wiper adjacent to the location of the applied force, in addition to contacting the conducting finger of the wiper at the precise location of the applied force. The result is that the voltage read across the wiper, and the position determined therefrom, is inaccurate. This problem is worsened if an overlay or laminating surface is used, since the force is further spread out. Traditional force sensing resistors and force and position sensing resistors in the prior art also require substantial calibration, and must account for such variables as temperature, humidity, and aging. In addition, the pressure-responsive resistance medium is expensive to use.
Thus, there is a need for an improved and inexpensive sensor for measuring the width and position of an object on the sensor accurately and conveniently without the need for calibration.
According to one embodiment, the present invention comprises a pressure sensor apparatus. The apparatus comprises a first electrode and a second electrode, each disposed on one of a pair of backing sheets; a resistive electrode, having a known electrical resistance, disposed on one of the pair of backing sheets so as to provide an electrical conduction between the first electrode and the second electrode; a shunt electrode, having a resistance that is lower than the resistance of the resistive electrode, disposed on one of the pair of backing sheets; and at least one spacer disposed so as to maintain the shunt electrode in a non-conductive arrangement relative to the resistive electrode, the shunt electrode configured such that when pressure acts thereon, the shunt electrode contacts the resistive electrode so as to provide an electrical conduction between the resistive electrode and the shunt electrode.
In accordance with one embodiment of the invention, the linear distance between two outer locations of pressure on the sensor corresponds to a change in resistance between the first and second electrodes. In accordance with another embodiment of the invention, the width of the object exerting pressure on the sensor corresponds to a change in resistance between the first and second electrodes. In accordance with another embodiment of the invention, the position of the object exerting pressure on the sensor corresponds to a change in resistance between the first and shunt electrodes.
According to another embodiment, the at least one spacer prevents the shunt electrode from contacting the resistive electrode except at a location of the applied force. In another embodiment, a plurality of spacers are disposed on the resistive electrode, while in another embodiment, a single spacer having windows is disposed on the resistive electrode.
According to another embodiment, the electrodes are disposed on one of the pair of backing sheets by printing, etching, silk-screening and spraying. According to another embodiment, the at least one spacer is disposed on one of the pair of backing sheets by printing, etching, silk-screening and spraying. In still another embodiment, the first, second and shunt electrodes are comprised of a material selected from a group consisting of copper, silver, aluminum and a conductive ink such as a graphite-based polymer thick film ink. In still another embodiment, the third electrode is comprised of a material selected from a group consisting of carbon and a non-conductive ink such as a titanium dioxide resistive ink. Preferably, at least one of the pair of backing sheets is thin and flexible, such as comprising a material selected from the group consisting of Mylar(trademark), Kapton(trademark) and polyester.